1. Field of the Invention
The present invention relates to a field excitation control system of the synchronous generator and, particularly, to that of the synchronous generator capable of supplying leading power factor reactive power with large capacity.
2. Description of the Prior Art
Generally, the synchronous generator employs a field excitation control system by which the field excitation flux (the field excitation current) is controlled so that the terminal voltage of the synchronous generator may be kept constant. More specifically, in the light of the fact that the increment of the field excitation flux is followed by the rise of the terminal voltage of the generator, the terminal voltage is fed back and compared with a predetermined set voltage, and the terminal voltage thereof is controlled in response to the difference generated by the comparison operation, so as to keep it constant. The field excitation control system designed so as to control the terminal voltage of the synchronous generator to be constant is called Automatic Voltage Regulator (AVR). An "AVR operation" to be used in this specification means the state in which the generator operates under the control by the AVR.
In recent cities, underground cable lines are finding increased utilization when laying down the transmission and distribution lines from the standpoint of restriction of the site and efforts to keep up an appearance of the streets. In this case, the capacitance is a problem in the power supply, the capacitance being formed between the cable and ground. That is, when the load is measured across the generator terminals, it takes the appearance of the leading power factor load. Accordingly, such cable lines have a tendancy to absorb the leading power factor reactive power. As a result, the generator connected to such cable lines is required to perform the condensive operation for supplying the leading power factor reactive power to the cable lines. However, in case the synchronous generator under control of the AVR, operates taking the leading power factor load, the increment of the leading power factor load is followed by the increment of the rotor angle of the synchronous generator, thus resulting in unstable operation thereof when the leading power factor load is increased. Therefore, the synchronous generator is given a restriction in increasing its compensative reactive power supply, when considering the stability of the generator operation. When the generator is operated supplying the reactive power exceeding such restriction, it is disturbed into stepping out of the system in the operation. In the future, it is anticipated there will be a greater increase in the employment of the underground cable lines. Thus, the need will increase for a new synchronous generator capable of stably supplying the sufficiently large reactive power in question even to the leading power factor load with a largest capacity. That the condensive operation, in particular in the light load region, tends to be restricted is inevitable, so far as the AVR is employed for the field excitation control system. This will be described in detail later.